properties: add "ambiguous_width" property for ambiguous East Asian Width (#270)

properties: add "ambiguous_width" property for ambiguous East Asian Width (#270)

Some characters have their width defined as "Ambiguous" in UAX#11.
These are typically rendered as single-width by modern monospace fonts,
and utf8proc correctly returns charwidth==1 for these.

However some applications might need to support older CJK fonts where
characters which where two-byte in legacy encodings were rendered as
double-width. An example of this is the 'ambiwidth' option of vim
and neovim which supports rendering in terminals using such wideness
rules.

Add an 'ambiguous_width' property to utf8proc_property_t for such characters.